fix: use unescaped %I in ntfy failure notification script

%i passes the escaped unit name which systemctl status cannot resolve,
causing "Failed to mangle name" errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/nixos/services/systemd-email-notify/default.nix

@@ -117,7 +117,7 @@ in
               ${cfg.ntfy.url}/${cfg.ntfy.topic}
           '';
         };
-        scriptArgs = "%i";
+        scriptArgs = "%I";
       };
     })
   ]);

systems/x86_64-linux/sgx/backup.nix

@@ -29,6 +29,10 @@ in
     ];
     backupPrepareCommand = ''
       HOME=/root ${pkgs.rsync}/bin/rsync -e "${pkgs.openssh}/bin/ssh"  --exclude-from /root/excludelist --no-specials --no-devices --numeric-ids --delete-after --partial -axz backup@mx.surfsite.org:/{etc,var,home,root} ${backup_new_path}
+      rc=$?
+      # exit code 24 = files vanished during transfer (normal for mail dirs)
+      if [ $rc -eq 24 ]; then exit 0; fi
+      exit $rc
     '';
   };